fix: interrupt idle worker waits during shutdown

backend/src/utils/interruptibleSleep.js

@@ -0,0 +1,25 @@
+/** A run owns its waits; cancelling wakes every waiter and clears its timer. */
+function createInterruptibleSleep() {
+  let cancelled = false;
+  const waiters = new Set();
+  return {
+    sleep(ms) {
+      if (cancelled) return Promise.resolve();
+      return new Promise(resolve => {
+        const finish = () => {
+          clearTimeout(timer);
+          waiters.delete(finish);
+          resolve();
+        };
+        const timer = setTimeout(finish, ms);
+        waiters.add(finish);
+      });
+    },
+    cancel() {
+      cancelled = true;
+      for (const finish of waiters) finish();
+    },
+  };
+}
+
+module.exports = { createInterruptibleSleep };